Solution for triangle

triangle/README.md

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+# Triangle
+
+Write a program that can tell you if a triangle is equilateral, isosceles, or scalene.
+
+The program should raise an error if the triangle cannot exist.
+
+Tests are provided, delete one `skip` at a time.
+
+## Hint
+
+The sum of the lengths of any two sides of a triangle always exceeds the
+length of the third side, a principle known as the _triangle
+inequality_.
+
+## Running tests
+
+```bash
+$ elixir bob_test.exs
+```
+
+(Replace `bob_test.exs` with the name of the test file.)
+
+## Source
+
+The Ruby Koans triangle project, parts 1 & 2 [view source](http://rubykoans.com)

triangle/triangle.exs

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+defmodule Triangle do
+  @type kind :: :equilateral | :isosceles | :scalene
+
+  @doc """
+  Return the kind of triangle of a triangle with 'a', 'b' and 'c' as lengths.
+  """
+  @spec kind(number, number, number) :: { :ok, kind } | { :error, String.t }
+  def kind(a, b, c) do
+    cond do
+      a <= 0 || b <= 0 || c <= 0  -> { :error, "all side lengths must be positive" }
+      a == b && b == c            -> { :ok, :equilateral }
+
+      (a == b || a == c || b == c) && valid_triangle?(Enum.sort([a,b,c]))  -> { :ok, :isosceles }
+      (a != b || a != c || b != c) && valid_triangle?(Enum.sort([a,b,c]))  -> { :ok, :scalene }
+      (a != b || a != c || b != c) && !valid_triangle?(Enum.sort([a,b,c])) -> { :error, "side lengths violate triangle inequality" }
+    end
+  end
+
+  defp valid_triangle?(l) do
+    (Enum.at(l,0) + Enum.at(l,1)) > Enum.at(l,2)
+  end
+end

triangle/triangle_test.exs

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+if System.get_env("EXERCISM_TEST_EXAMPLES") do
+  Code.load_file("example.exs")
+else
+  Code.load_file("triangle.exs")
+end
+
+ExUnit.start
+
+defmodule TriangleTest do
+  use ExUnit.Case, async: true
+
+  test "equilateral triangles have equal sides" do
+    assert Triangle.kind(2, 2, 2) == { :ok, :equilateral }
+  end
+
+  test "larger equilateral triangles also have equal sides" do
+    assert Triangle.kind(10, 10, 10) == { :ok, :equilateral }
+  end
+
+  test "isosceles triangles have last two sides equal" do
+    assert Triangle.kind(3, 4, 4) == { :ok, :isosceles }
+  end
+
+  test "isosceles triangles have first and last sides equal" do
+    assert Triangle.kind(4, 3, 4) == { :ok, :isosceles }
+  end
+
+  test "isosceles triangles have two first sides equal" do
+    assert Triangle.kind(4, 4, 3) == { :ok, :isosceles }
+  end
+
+  test "isosceles triangles have in fact exactly two sides equal" do
+    assert Triangle.kind(10, 10, 2) == { :ok, :isosceles }
+  end
+
+  test "scalene triangles have no equal sides" do
+    assert Triangle.kind(3, 4, 5) == { :ok, :scalene }
+  end
+
+  test "scalene triangles have no equal sides at a larger scale too" do
+    assert Triangle.kind(10, 11, 12) == { :ok, :scalene }
+  end
+
+  test "scalene triangles have no equal sides in descending order either" do
+    assert Triangle.kind(5, 4, 2) == { :ok, :scalene }
+  end
+
+  test "very small triangles are legal" do
+    assert Triangle.kind(0.4, 0.6, 0.3) == { :ok, :scalene }
+  end
+
+  test "triangles with no size are illegal" do
+    assert Triangle.kind(0, 0, 0) == { :error, "all side lengths must be positive" }
+  end
+
+  test "triangles with negative sides are illegal" do
+    assert Triangle.kind(3, 4, -5) == { :error, "all side lengths must be positive" }
+  end
+
+  test "triangles violating triangle inequality are illegal" do
+    assert Triangle.kind(1, 1, 3) == { :error, "side lengths violate triangle inequality" }
+  end
+
+  test "triangles violating triangle inequality are illegal 2" do
+    assert Triangle.kind(2, 4, 2) == { :error, "side lengths violate triangle inequality" }
+  end
+
+  test "triangles violating triangle inequality are illegal 3" do
+    assert Triangle.kind(7, 3, 2) == { :error, "side lengths violate triangle inequality" }
+  end
+end
+